Zinc-plated material, such as zinc-plated steel sheet or coil, is vulnerable to corrosion caused by corrosion medium in ambient environment during production, storage, transportation, and application process. For preventing the corrosion of the zinc-plated material, the treating agent containing Cr6+ like chromate is widely adopted to form a chromate passivation film on the zinc-plated steel sheet, and then an anticorrosive coat is formed by heat curing.
But EU RoHs Directive coming into effect on 1 Jul. 2006 bans product with hazard substance (such as Pb, Cr6+, Hg, Cd, polybrominated biphenyl (PBB), and polybrominated diphenyl ethers (PBDE)) content beyond standard. To satisfy the requirement, global steel enterprises have adopted surface thin coat treatment technique for the zinc-plated steel sheet, such as non-hazardous passivation treatment. The available non-hazardous passivation treatment liquid comprises Nihon Parkerizing LSI-C-2011 and Henkel 6000.
CN 1268583A disclosed a Cr-free passivation solution for protection of zinc-plating layer and coating method thereof, the method for preparing the passivation solution comprises (1) mixing water soluble acrylate resin and water at volume ratio of 1:1-4 for dilution; (2) adding 1-5 g of molybdate per liter of the diluted water-soluble acrylate resin solution to give colorless transparent passivation solution with pH of 6-9. The passivation solution is uniformly coated on the zinc-plated part surface.
The aforementioned method can improve the corrosion resistance of the zinc-plated part to some extent; but the treated zinc plated steel sheet still has the disadvantage of poor water resistance, and generates white powder on its surface in high-temperature and high humidity environment, or blushing and chalking occur 24 hr after 100° C. hot water is dripped on the zinc-plated steel sheet at room temperature.
Therefore, the passivation coat formed by the available Cr-free passivation solution for the zinc-plated material has the shortcomings of poor water resistance and poor high temperature resistance.